The present invention relates to a signal transmission system and, more particularly to a pilot signal transmission system in which two terminal stations transmit signals including link monitoring pilot signals (referred to as pilot signals hereunder) with carrier waves to each other either directly or through a repeater station.
In general, a radio terminal station (terminal station) is composed of a radio transmitting section (transmitting section), a radio receiving section (receiving section), a pilot oscillating section for generating a pilot signal (transmitting pilot signal) to be fed to said transmitting section, a transmitting pilot signal detecting section for detecting the output of said pilot oscillating section, a pilot filtering section for filtering a pilot signal (receiving pilot signal) included in the signal demodulated in said receiving section, and a receiving pilot detecting section for detecting the output level of said pilot filtering section. In such a terminal station, it is a common practice to provide either only one pilot oscillating section to be used currently (examples of this type are FM72-400 Radio Equipment manufactured by Siemens Aktiengesellschaft and CTR145C Signal Tranceiver manufactured by GTE International Incorporation) or two pilot oscillating sections, one for current use and the other for standby.
On the other hand, a radio baseband repeater station (repeater station) is generally composed of, for the up- and down-links, two transmitting sections, two receiving sections, two receiving pilot filtering sections for filtering receiving pilot signals, and two receiving pilot detecting sections for detecting output levels of said pilot filtering sections. Conventional systems employed in this repeater station for repeating the signals demodulated to the transmitting sections include the one in which the demodulated signals is repeatered after the pilot signals contained therein have been removed by pilot removing sections, and another one in which the demodulated signals (containing the pilot signals) are repeatered themselves.
What is common to the terminal station and the repeater station in the prior art is that the transmitting pilot signal, the receiving pilot signal and the pilot signals on the up- and down-links are handled independently of each other.
The aforementioned conventional terminal station has disadvantages in that if there is provided only one pilot oscillating section to be used currently, it will become impossible to monitor the transmission path upon fault of the pilot oscillating section, and if two pilot oscillating sections are provided with one of them for standby, the apparatus as a whole is costly to manufacture and requires a greater space to install.
The above-described conventional repeater station also has disadvantages in that in the case where the demodulated signals are repeatered after the pilot signals have been eliminated, pilot eliminating sections are necessitated so that one or two pilot oscillating sections for feeding transmitting pilot signals to the transmitting sections for the up- and down-links become necessary, while in the case where the demodulated signals are repeatered in themselves, it will become impossible to monitor a transmission path upon fault of a pilot oscillating section in a terminal station or upon fault of a receiving section in the repeater station, so that as a standby source of the pilot signals which are normally transmitted from the terminal stations via the up- and down-links, a pilot oscillating section must be provided in the repeater station.